The first three Gospels (Matthew, Mark and Luke) are called the Synoptic Gospels. The term synoptic comes from the words meaning "same" (syn) and "sight" (optic), or "to see together". There is quite a bit of overlap with what these three books report, while about 90% of the Gospel of John contains different information.
There are different views on how Christians should view the overlapping information. On the one hand, if the authors are...
Christianity is based on historical events. Throughout the Bible, there were eyewitnesses to events, and those people reported what happened. The Gospels, in particular, have extra-Biblical sources which help confirm their accuracy. First century non-Christian historians who refer to events in the Gospels include Josephus, Pliny the Younger, Suetonius, Tacitus, Thallus, and Emperor Trajan.